This invention relates to a detector for detecting noise included in a signal, and a device for carrying out noise reduction by using this noise detector.
In the case where a television, a VTR, or audio equipment, etc. carries out signal processing, noise is apt to take place particularly when recording and reproducing a signal. It was conventionally extremely difficult to theoretically discriminate between an original (primary) signal and a noise signal. For this reason, there did not exist a device for automatically detecting whether or not there is a noise signal, or to what extent a noise signal is included. Accordingly, the human being could only judge a reproduced voice or picture (pictorial image), etc. by natural senses.
In an apparatus for processing a video signal or an audio signal, etc., a noise reduction device is provided. Explanation will be given by taking an example of a conventional noise reduction device used in a VTR. As the representative of the noise reduction device, there are a noise canceller, a non-recursive comb filter, and a recursive comb filter. Their configurations are shown in FIGS. 1 to 3, respectively.
The noise canceller has a configuration as shown in FIG. 1. A signal inputted from an input terminal 41 is passed through a high-pass filter 51, a limiter 52 and a low-pass filter 53. In a predetermined frequency band determined by the high-pass filter 51 and the low-pass filter 53, the component of a very small level determined by the limiter 52 is extracted. This component is regarded as a noise signal. Such a noise signal is subtracted from the input signal at a subtracter 44 to output a signal indicating a difference therebetween from an output terminal 42. Generally, a frequency band which is disturbing the human eye is considered to be a frequency band in the vicinity of 1 MHz. Accordingly, an approach to eliminate very small components in this band is carried out.
The non-recursive comb filter serves to suppress noise signals generated at random by using two signals correlative with each other, and has a configuration as shown in FIG. 2. A signal having correlation only for any one of one horizontal interval (time period), one field time period and one frame time period, like a VTR signal is delayed by a delay unit 61. A signal inputted from input terminal 41 is added to that delayed signal at an adder 63. A signal outputted from the adder 63 is then multiplied by 1/2 at an amplifier 65 so that its signal level is in correspondence with the original signal level for a second time. The signal thus obtained is outputted from output terminal 42. By using two signals correlative with each other, suppression can be made such that the level of noise generated at random without correlation becomes equal to (1/2).sup.0.5.
The recursive comb filter also serves to suppress the influence of a noise signal by using two signals correlative with each other, and has a configuration as shown in FIG. 3. A signal inputted from input terminal 41 is multiplied by (1-K ) (0&lt;K&lt;1) by an amplifier 71. Further, another signal delayed by any of the above-described time periods at a delay circuit 74 is multiplied by K by an amplifier 75. A signal outputted from the amplifier 75 and a signal outputted from the amplifier 71 are added at an adder 72. An added signal thus obtained is outputted from output terminal 42.
However, even in the case where any noise reduction device is used, there was the problem that the picture quality of a reproduced picture is degraded. For example, in the noise canceller device, not only a noise signal but also an original signal component is included as very small component in a predetermined frequency band extracted by the high-pass filter 51, the limiter 52 and the low-pass filter 53. For this reason, the original signal component is subtracted from a signal together with the noise signal, resulting in lowered resolution.
In addition, in the case where the non-recursive filter or the recursive filter is used, such filter operates in the same manner also with respect to two signals which are not correlative with each other. For this reason, sagging or after-image may take place in a vertical direction of a picture.
Using these filters with respect to a signal having low SN ratio is effective for improvement in SN ratio. However, in the prior art, as described above, there was no device for automatically detecting whether or not there is a noise signal included in a signal, or to what extent such a noise signal is included therein. For this reason, noise reduction would be implemented to a signal already having a high SN ratio and does not strongly need noise reduction, leading to the adverse effect of lowering the resolution.
As stated above, there was in the prior art no device for automatically detecting whether or not there is a noise signal included in a signal, or to what extent such noise signal is included therein, so a human being could only judge such state of noise signal by eye or ear.
In addition, since the conventional noise reduction device does not include a function to automatically detect whether or not there is a noise signal, or to what extent such noise signal is included therein, noise reduction would be implemented to all signals irrespective of SN ratio, leading to the adverse effect that the picture quality or the sound quality is lowered.